This invention relates to optical devices for surveying, rangefinding, and the like, and is particularly adaptable for telescopes and their use in aiming firearms. The invention will be described in a riflescope embodiment.
Since the advent of firearms, ways and means have been sought to accurately aim these devices at targets of interest. Evolving from simple notch-and-bead arrangements, the optical riflescope has proved to be the most effective device for the fulfillment of this task.
Riflescopes have been commercially available for many years and have remained largely unchanged since their introduction. These instruments comprise two basic types. The simplest form comprises a body tube, objective and ocular lenses disposed at essentially opposite ends thereof, and an erector lens assembly situated within the body tube in the space between the objective and ocular lenses. Without the erector optics the image formed by the objective lens would be inverted as in cellestial telescopes. The inversion of the images of cellestial objects is of no consequence but telescopes adapted for use in the aiming of firearms require the inclusion of erector optics in order to provide an image which will appear normal. The erector optics are mounted within a secondary tube which is itself mounted within the main body tube of the riflescope. The erector optics tube is usually controllably pivotable by means of adjustment screws for the positional fine adjustment of the target image relative to an essentially stationary sighting reference. This reference usually consists of a pair of aiming crosshairs or a reticle disposed in the riflescope either at the focal plane of the objective, in front of the erector lens assembly, or at the rear plane of focus shared by the erector and the ocular optical elements.
In a second and somewhat more complex form of riflescope, the erector optics are also controllably movable along the optical axis within their tube in such a manner as to cause the focal length of the optical system to vary to provide an image of variable size. A ring external of the body tube is manipulated by the shooter to alter, at will, the size of the image formed by the riflescope within the limits afforded by the design and structure of the particular device. This operation is typically called "zooming" and a range of from three to nine power is common. At three power the image formed by the device is three times lifesize while at nine power it is nine times lifesize.
The body tube, or housing, is provided with mounting means by which the riflescope is affixed to the firearm. Once mounted, the riflescope is "zeroed", an operation which establishes a precise relationship between the bore line of the firearm and the operator's visual line of sight through the riflescope. This adjustment is facilitated by the previously mentioned screw means constituting a horizontal (windage) adjustment and a vertical (elevation) adjustment. "Zeroing" is done in relation to an impact point at a known distance down range. For instance, a firearm may be "zeroed" so that the aiming reference, the reticle or crosshair intersection, is aligned with a point one hundred (100) meters distant and is coincident with the placement or location of the projectile at that range. Once established, the alignment is permanent until such time as the rigors of field use necessitate the re-establishment of alignment by zeroing anew.
In riflescopes of basic construction, the range to targets must be compensated for entirely by the shooter who first estimates a target's range and then imposes a degree of holdover deemed to be compensatory of projectile drop at that distance. However, few shooters become adept at estimating range and in compensating properly for bullet drop. In the field, variations in terrain, air density, and elevation can cause even the best of shooters to misjudge target range a significant percentage of the time resulting in their often being well off the mark when firing. And shooters who are expert at such judgment are able to attain accuracy with only one or two specific cartridges and rifles with which they have become familiar by practice. With increasing target distance the expert shooter aims ever higher in estimated increments of the target's image size: quarter height of target over, half height over, full height over, and so forth. This judgment is based directly on the size of the target image in the riflescope and, stated in the simples possible way, the shooter knows that as down range distance increases target image size decreases.
Limited static and dynamic means have evolved for range determination and aiming compensation based on these facts.